az keyvault certificate show --vault <vault> -n <certName> --query "x509ThumbprintHex" -o tsv

Pulumi.Command

var getThumb = new Command("getThumb", new CommandArgs
{
    Create = "az keyvault certificate show --vault $VAULT -n $NAME --query \"x509ThumbprintHex\" -o tsv",
    Environment =
    {
        { "NAME", certName },
        { "VAULT", vault.Name },
    },
});